The invention relates to solid oxide fuel cells and more particularly to a method of fabricating the manifolds therefor. A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. In a fuel cell, cathode material defines the passageways for the oxidant and anode material defines the passageways for the fuel, and an electrolyte separates the cathode and anode materials. The fuel and oxidant fluids, typically gases, are continuously passed through separate cell passageways. The fuel and oxidant discharges from the fuel cell generally remove the reaction products and heat generated in the cell. The fuel and oxidant are the working fluids and as such are not considered an integral part of the fuel cell.
The type of fuel cell for which this invention has direct applicability is known as the solid electrolyte or solid oxide fuel cell, where the electrolyte is in solid form in the fuel cell. In the solid oxide fuel cell, hydrogen or a hydrocarbon fuel is preferably used as the fuel and oxygen or air is used as the oxidant, and the operating temperature of the fuel cell is between 700.degree. C. and 1,100.degree. C. The hydrogen passing through the fuel cell reacts with oxide ions on the anode to yield water, which is carried off in the fuel flow stream, with the release of electrons into the anode material. The oxygen reacts with the electrons on the cathode surface to form the oxide ions which then pass into the electrolyte material. Electrons flow from the anode through an appropriate external load to the cathode, and the circuit is closed internally by the transport of oxide ions through the electrolyte. The reaction process is well known and more thoroughly delineated in U.S. Pat. Nos. 4,499,663 and 4,816,036.
In practice, fuel cells are not operated as single units; rather, they are stacked in series. In a stack of cells, an interconnect connects anode of one cell to cathode of the next in electrical series to build voltage. Various configurations of fuel cells are depicted in U.S. Pat. Nos. 4,476,198 (Ackerman et al); 4,476,196 (Poeppel et al); 4,753,857 (Hosaka); 4,769,297 (Reiser et al): 4,770,955 (Ruhl); and 4,510,212 (Fraioli).
Cellular type fuel cell cores (see U.S. Pat. No. 4,476,198) of the prior art are made by the process whereby the compositions used for the four materials are put into four distinct slurries. Each slurry is then placed in a reservoir of a squeegee-type device which is pulled over a flat surface and hardens or plasticizes into a layer of the material having the desired thickness. In this manner the electrolyte wall or interconnect wall is formed by a first layer of anode material followed by a layer of either electrode or interconnect material and finally by a layer of the cathode material. The layers are bonded together since the binder system is the same in each layer.
Related U.S. Pat. No. 4,816,036 (Kotchick) teaches another method of forming a cellular core, whereby the compositions for the four materials are individually mixed to a plastic consistency and subsequently hot rolled into thin sheets. The thin sheets can then be hot rolled into multilayer tapes, formed, stacked, and fired as a monolith to produce the fuel cell with integral fuel and oxidant manifolding.
It should be particularly noted that the assemblies of the prior art references seek to construct fuel cells having multiple, stacked individual cells. The problem of manifolding the respective fuel and oxidant gases to the anode and cathode surfaces is generally ignored. In addition, external gas manifolds are often proposed which are fabricated separately from the fuel cell core. External gas manifolds thus require attachment to the fuel cell core and add stringent tolerance requirements for stack hardware design. Therefore, a fuel cell core design with integral gas manifolds is desirable.